1. Field of the Invention
The field of the present invention is internal combustion engines for motor vehicles and, in particular, utilization of the heat energy normally discarded in the exhaust of internal combustion engines by converting the heat to mechanical work in a highly efficient manner, thereby increasing the overall efficiency of fuel utilization.
2. Prior Art
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide.
Internal combustion engines create mechanical work from fuel energy by combusting the fuel over a thermodynamic cycle consisting typically of compression, ignition, expansion, and exhaust. Expansion is the process in which high pressures created by combustion are deployed against a piston, converting part of the released fuel energy to mechanical work. The efficiency of this process is determined in part by the thermodynamic efficiency of the cycle, which is determined in part by the final pressure and temperature to which the combusted mixture can be expanded while performing work on the moving piston.
Generally speaking, the lower the pressure and temperature reached at the end of the expansion stroke, the greater the amount of work that has been extracted. In conventional engine designs, expansion is limited by the fixed maximum volume of the cylinder, since there is only a finite volume available in which combusting gases may expand and still perform work on the piston. This makes it impractical to expand to anywhere near ambient temperature and pressure, and instead a large amount of energy remains and is normally discarded with the exhaust. The production of work from the initial expansion of combustion gases is commonly referred to as xe2x80x9ctopping,xe2x80x9d while the extraction of work from once-expanded gases is referred to as a xe2x80x9cbottoming cycle.xe2x80x9d
Bottoming cycles are commonly employed as part of the combined cycle operation of steam power plants. xe2x80x9cPerformance Analysis of Gas Turbine Air-Bottoming Combined System,xe2x80x9d Energy Conversion Management, vol. 37, no. 4, pp. 399-403, 1996; and xe2x80x9cAir Bottoming Cycle: Use of Gas Turbine Waste Heat for Power Generation,xe2x80x9d ASME Journal of Enqineering for Gas Turbines and Power, vol. 118, pp. 359-368, April 1996 are representative of the state of the art in this field. Exhaust heat rejected from a primary gas turbine (the topping cycle) is used to heat water to produce steam that is expanded in a secondary steam turbine (the bottoming cycle) Although in this case the working fluid of the bottoming cycle is steam, other fluids having more favorable physical or thermodynamic properties may be used, for instance ammonia-water mixtures or even a gas.
Bottoming cycles that employ water/steam or any other recirculating medium as the working fluid must provide additional hardware for recirculation and purification. For instance, steam-based plants require a boiler, a sophisticated steam turbine, condenser, purification system to prevent mineral deposits and scaling, pumps, etc. For this reason, they are practically limited to stationary applications such as public power utilities and industrial plant use and are precluded from mobile applications such as motor vehicles.
Motor vehicles represent a large portion of total energy use in the world today. There are, of course, differences between stationary power plants and power plants of motor vehicles. First, motor vehicles usually do not employ a turbine in the topping phase and so produce a less uniform flow rate of gases in the exhaust. Second, for a motor vehicle the equipment devoted to the bottoming cycle should be low cost, relatively simple to operate and maintain, and lightweight. Third, in a motor vehicle the working fluid of the bottoming cycle should be safe and not require extensive recirculation hardware.
The use of air as a working fluid for stationary power generating applications has been investigated. In U.S. Pat. No 4,751,814, xe2x80x9cAir Cycle Thermodynamic Conversion System,xe2x80x9d a gas turbine topping cycle is combined with an air turbine bottoming cycle. Air is compressed in an intercooled multi-stage compression system that maintains air temperature as low as possible. Heat from the turbine exhaust is transferred to the compressed air via a counter flow heat exchanger, and the heated compressed air is expanded through an air turbine to provide at least sufficient work to run the compressors and preferably enough to use for other purposes. This system obviates sophisticated purification and processing of the working fluid (atmospheric air) if it is recirculated at all, and dispenses with bulky steam handling equipment. However, the system depends on turbine-based topping and bottoming apparatus which is not well suited to conventional motor vehicle applications.
Piston (or other means with sealed moving surfaces) compressors and expanders provide high efficiency for the processes of compression and expansion, but exhibit friction that is generally higher than a gas turbine of the same size (i.e., operating at similar gas flow rates). However, gas turbines (especially for the smaller sizes that would be needed for road vehicles) do not provide process efficiency as high as desired because of gas leakage around the edges of the turbine blades (the moving surfaces), which are not sealed.
Further, gas turbines operate at extremely high speed (often greater than 100,000 RPM), and the speed reduction gearing necessary to provide mechanical power at speeds usable in a mobile vehicle (e.g., less than 6,000 RPM) is costly and inefficient.
Therefore, an object of this invention is to provide a power train inclusive of a bottoming cycle which is suitable for use in automobiles.
Another object of the present invention is to provide such a power train using air as a working fluid in the bottoming cycle.
Yet another object of this invention is to provide a sealed moving surface compressor and expander design that performs compression and expansion with minimal friction, so that the net efficiency is significantly greater than that achievable with gas turbines.
A further object of this invention is to provide compressor and expander designs that operate efficiently at speeds below 6,000 RPM.
Accordingly, the present invention provides an air bottoming power train which includes a source of combustion exhaust gas, e.g. the internal combustion engine (ICE) of an automobile; a compressor which receives a gaseous working fluid and compresses to an elevated pressure; a cooler for cooling the compressor to provide near isothermal compression; an expander having a plurality of cylinders, each cylinder having a piston reciprocally mounted therein and operating in a two stroke cycle including an expansion stroke and an exhaust stroke, the pistons driving an output shaft; a compressed gas line for feeding the compressed gaseous working fluid from the compressor to the expander; and an expander valve for successively admitting the compressed gaseous working fluid from the compressed gas line into individual cylinders of said expander in succession and for continuously admitting the compressed gaseous working fluid to an individual cylinder through a first portion of the expansion stroke to maintain constant pressure. A heat exchanger is located in the compressed gas line for indirect heat exchange between the compressed gaseous working fluid and the exhaust gas, and is fed the exhaust gas by an exhaust gas line running through the heat exchanger.
A preferred expander includes a cylinder barrel with a plurality of cylinders formed in a circle within the cylinder barrel, open at one end face of the cylinder barrel and closed at an opposite endface of the cylinder barrel. A valve plate seals closed the one end of the cylinder barrel. The valve plate has a compressed gas inlet and an exhaust gas outlet. The cylinder barrel and the valve plate are mounted for relative rotation therebetween, the relative rotation serving to drive an output shaft. The expander preferably has a bent-shaft configuration, and has a total displacement which changes as an angle between the cylinder barrel and the output shaft is changed. The valve plate my have an arcuate groove in a face sealing against said cylinder barrel, the arcuate groove being in communication with the exhaust gas outlet and in register with the circle.
A second preferred embodiment of the expander is a Scotch yoke piston motor including plural paired and axially aligned cylinders on opposing sides of an output shaft and pistons reciprocally mounted in the cylinders and drivably connected to the output shaft. Each cylinder is axially divided into a thermally insulated outer portion and a cooled inner portion, the insulated outer portion being separated from the cooled inner portion by a thermal brake; and further, each piston is axially divided into a hollow outer and a cooled inner section, the cooled inner section having an exterior surface bearing oil rings sealing with the cooled inner portion of the cylinder, the hollow outer section being thermally isolated from the cooled inner section by a thermal brake.
The present invention utilizes an air bottoming cycle in conjunction with unique multi-cylinder piston compressor and expander designs that are well suited for use with the conventional automotive exhaust gas stream.
An ideal representation of the desired air bottoming thermodynamic cycle is shown in FIG. 1. The line ab represents intake of working fluid to the compressor. Line bc represents isothermal compression of the working fluid. Line cd represents absorption of heat by the working fluid at constant pressure during constant pressure expansion. Line db represents adiabatic expansion of the heated compressed gas to ambient conditions, producing the maximum possible work. Line ba represents the exhaust of the expanded air before the beginning of the next cycle.
The present invention effects an air bottoming cycle consisting of five distinct phases: (1) Compression, made relatively isothermal by cooling, of a gaseous working fluid such as air in a compressor, and optional buffering of the compressed air stream in an optional surge tank to reduce fluctuations in the heat exchanger inlet stream; (2) Addition of heat to the compressed working fluid at relatively constant pressure through a device such as a counter flow heat exchanger recovering heat from the internal combustion engine exhaust; (3) An initial, near constant pressure, expansion of the heated, compressed working fluid; (4) A final relatively adiabatic expansion of the partially expanded working fluid to as close to ambient conditions as possible, producing the maximum amount of work and; (5) Exhaust of the expanded working fluid from the expander or its conveyance to an appropriate destination such as the air intake of the internal combustion engine.
The cooled compressor performs a relatively isothermal compression of a working fluid such as air, which should be at the lowest practical temperature before entry to the heat exchanger in order to maximize the potential for recovery of heat. Near isothermal compression is achieved by one or more of the following means: cooling the compressor chamber walls using a water-based coolant, air or other fluid coolant; increasing the turbulence of the intake working fluid to increase the heat transfer coefficient and in-chamber mixing; increasing the roughness of the chamber walls to increase boundary layer turbulence and thus heat transfer coefficient and to increase heat transfer area; an oil jet spray to the bottom of each piston; and injecting a liquid into the compressing working fluid to extract heat from compression through phase change (evaporation) of the injected liquid. One unique feature of the present invention is the option of injecting the liquid fuel (to assist in cooling the compressing air) that, being mixed with the exhausted air at the end of the bottoming cycle, will subsequently be routed to the combustion engine which supplies the hot exhaust gas to xe2x80x9cfuelxe2x80x9d this bottoming engine. Methanol or ethanol are particularly good fuels for this use since they both can be easily mixed with water to provide an optimum mixture.
The compressed working fluid is passed through the optional surge tank and into the counter flow heat exchanger. The working fluid experiences a temperature increase, adding energy to the already compressed gas. Relatively constant pressure is assured because the heated, compressed working fluid enters the expansion chamber at a rate equal to the propensity for the heat to raise the pressure of the gas, and thus an initial constant pressure expansion phase is achieved. After the intake valve is closed, expansion continues to the end of the expansion stroke, producing mechanical work as it expands. The near-ambient pressure air exhausted by the expander could be released to the atmosphere or optionally fed to the air intake of the internal combustion engine. Optionally, the exhausted gas from the expander can be fed to the intake of the internal combustion engine (at any boost pressure) through the xe2x80x9cPhase Change Heat Enginexe2x80x9d which increases the efficiency of the overall cycle and serves as an intercooler for the charge air of the internal combustion engine. The exhaust gas could also be the source of heat energy for a xe2x80x9cPhase Change Heat Enginexe2x80x9d incorporated into yet another integrated configuration. The xe2x80x9cPhase Change Heat Enginexe2x80x9d is disclosed in my copending application filed on even date herewith, the teachings of which are incorporated herein by reference.
Use of a surge tank allows the use of fewer pistons in the compressor by moderating fluctuations in the compressor outlet stream and tends to reduce temperature increase during each compression stroke.